Ehh, I dunno. At first glance it may seem slimy, but really, you're paying for the right to choose your seat in a premium seating area, versus allowing United to seat you where they have room. You could easily have ended up in middle seats in the back, which was the chance you took when you decided to keep your $40. Paying more gives you more control.

It's similar to the way upgrades work: You can take a chance that your upgrade will clear at the 24/48/100-hour mark, or you can buy a First Class seat and guarantee that you'll ride up front. Some people find that certainty worth paying for; some don't.